This invention relates to television and is directed to method and apparatus for improving the sharpness of television pictures. The reproduction of geometrical details is an important problem in television. Many factors are involved, but when the number of scanning lines per frame and the number of frames per second have been fixed, the horizontal and vertical resolutions are largely a function of system bandwidth. As such, a bandwidth is perfectly defined. In a given standard, measured horizontal and vertical resolutions are finite and well established.
Resolution in the horizontal domain is frequently expressed and understood in terms of maximum signal bandwidth, while resolution in the vertical domain is a function of the number of scan line samples over which the vertical transition is manifested, the sampling being at the line scan repetition rate. Since a television picture display occupies a given two-dimensional space, picture resolution is ultimately measured by the amount of display space in the picture's spatial dimension which is required for or occupied by the picture transition. This space is frequently referred to as "rise time". The less space required for rise time, the sharper the picture will appear. Thus, when considered in terms of shortened rise times, improved picture resolution transcends the differences otherwise existing between the horizontal domain and the vertical domains.
For example, the NTSC standard specifies a 525 line per frame scanning rate, and a 4.2 MHz luminance bandwidth. In these specified conditions, the vertical resolution cannot exceed 240 scan lines, while the horizontal resolution is limited to the equivalent of approximately 330 scan lines. These resolution limits were perfectly adequate when the picture displays were small and of limited sharpness. However, today, with the arrival of high precision, bright and well focused large screen displays (such as television projection systems), these limits are no longer adequate, and it is necessary to give to the viewer the appearance of a high resolution picture without increasing the bandwidth requirements constraining the television picture source.
Many attempts have been made in the prior art to increase the apparent sharpness of a television picture without increasing bandwidth throughout the television system.
A first type of device known in the art for increasing apparent picture resolution to some extent was image enhancers and aperture equalization systems. Those systems are described, for example, by A. N. Thiele in an article entitled "Horizontal Aperture Equalization,", Radio and Electronic Engineer, Vol. 40, No. 4, October 1970, page 193; and in CBS Laboratories Mark IV Automatic Image Enhancer Technical Bulletin, March 1974, and Philips Colour Telecine System brochure, page 25-27. The devices and methods described in these references generally operated in a strictly linear fashion. A high frequency signal was derived from the input transition and was thereafter added back to the transition without envelope delay error and with proper phasing. In those conditions the resultant output transition was apparently shorter in duration than the input transition. However, those prior approaches had at least two drawbacks.
A first drawback was that noise, as well as the signal transition, became enhanced in the process. In order to avoid this undesirable result, some prior devices employed an amplitude threshold in the enhancement path. Unfortunately, for low level transition signals below the threshold, there would be no enhancement: i.e., no apparent improvement in low contrast picture details.
A second drawback was that transition sharpening could not be obtained with those approaches without adding "preshoot" and "overshoot" which are directly analogous to pre-ringing and post-ringing artifacts surrounding the original transition. While actual duration of a transition did not vary, the passage from black to white did not happen in the same fashion. The visible transition zone on the display screen was as wide as before, but objects being displayed seemed to be surrounded with black and white margins. Thus, the preshoot and overshoot artifacts gave the resultant picture display an unnatural or cartoon-like appearance.
In order to avoid introducing visible overshoots, the present inventor presented a technique which defeated, in practice, the effect of aperture correction for large transitions. These improvements are described in the present inventor's prior U.S. Pat. Nos. 4,262,304 and 4,847,681. While visible overshoots disappeared with the patented approaches, large magnitude horizontal and vertical transitions in the picture image were no longer sharpened.
A second type of device found in the prior art is a video "crispener". A crispener is a transition processing device which generates nonlinearly shortened duration enhancement components derived from the original picture transition and combines the enhancement components with the transition without introducing preshoot and overshoot. The video crispener may thus be thought of as a picture signal spectrum expander. Examples of prior art video crispeners are described in U.S. Pat. No. 2,740,071 to Goldmark and Reeves, U.S. Pat. No. 2,851,522 to Hollywood, and in an article by Goldmark and Hollywood entitled "A New Technique for Improving the Sharpness of Television Pictures". Proceedings of the IRE, October 1951, p. 1314. The present inventor presented an improvement in video crispening techniques in his prior U.S. Pat. No. 4,030,121.
As video crispeners act, in effect, as spectrum expanders (or the equivalent thereof in the vertical domain), they achieve a reduction of the transition duration (rise time) without introducing preshoot and overshoot. However, video crispeners are relatively inefficient in improving the sharpness of small picture details resulting from small transitional levels in the picture signal.
A hitherto unsolved need has therefore arisen for an enhancement technique which reduces the rise time of a horizontal or vertical transition without regard to its transitional level and which does not introduce any visible image artifacts, such as noise, overshoots, ringing and the like.